The present invention relates to an X-ray tube cathode multiple polarization device and to a radiation source incorporating such a device. It is intended for use in radiology and more particularly for multifocus tubes. It makes it possible in an advantageous manner to reduce the number of conductors in the cable for supplying high voltage to the sheath and increases the flexibility of use of radiation sources.
In radiology, it has been found that the obtaining of good images, both in conventional image production and in image production by reconstruction assisted by a computer, largely depends on the shape and dimensions of the radiation emitter focus. One widely used tube type comprises two selectively switched filaments for producing a large or a small focus as a function of the polarization.
In radiology, it is often necessary to have more dimensions available. For this purpose, it has been proposed to provide the cathode with focusing electrodes raised to the cathode potential. It has also been proposed to mechanically regulate the length of the filament. It has also been proposed to arrange perpendicular to the filament axis and on either side of the latter, two concentrating parts, which are electrically insulated from the shell and raised to a potential regulatable as a function of the desired length. Finally, it has been proposed in the latter case to have two filaments with a common centre polarization terminal. One of these is added to the other for varying the length of the focus by step, which limits for example, the variation of the concentration voltage of the aforementioned parts.
The polarization of these various auxiliary members is ensured by a cable having a plurality of conductors from the high voltage generator to the tube or radiation source, which is often at a distance of several meters. This is disadvantageous from two standpoints. Firstly, a cable with a plurality of conductors increases in cost with the number of conductors and their number is limited on the presently proposed equipment. In addition, the length and number of conductors in a cable increases the reactive part of the cable impedance. This leads to a by no means negligible pulse transmission time, when it is a question of operating a tube at a high speed, as is the case in X-ray cinematography or X-ray scanning. Furthermore, stray capacitances cause, when switching on, a sudden drop in the polarization voltages and consequently a loss of dimensional control of the foci.